Typically the final operation in the production of electromagnetic actuators involves inter-positioning and affixing together two main portions: a stationary mainframe portion and a vibratable coil assembly. The mainframe portion includes a magnetic system having a magnet, yoke and polepieces defining a gap, all rigidly incorporated with the mainframe as a common mass. The gap is typically formed in an annular shape between a cylindrical pole piece and a surrounding polepiece that defines a circular opening. The coil assembly includes a tubular coil, known in a loudspeaker as the voice coil, typically wound on an insulating bobbin and located in the magnetic gap, and also includes suspension means for providing lateral constraint to keep the coil concentric in the gap while leaving it free to vibrate axially. Typically a suspension member known as a spider, attached around the coil form, provides a peripheral attachment region that is designed to be adhesively bonded to a corresponding portion of the mainframe in the final assembly process. In a typical loudspeaker there is also a second suspension member known as a surround, attached around the outer edge of the conical diaphragm; the surround has a peripheral attachment region that is designed to be adhesively bonded to a surrounding region of the mainframe in the final assembly process.
In manufacturing the actuator, the coil assembly is initially positioned to place the coil and its bobbin in the magnetic gap: it is then positioned as accurately as possible for concentricity, then held in this optimal location during the curing cycle of the adhesive bonding agent applied between the attachment regions of the suspension member(s) and the mainframe attachment region(s). The accuracy of this final positioning operation is critical for the performance and reliability of the actuator since the concentricity becomes fixed upon bonding, with no way of subsequent readjustment. Sufficient clearance margin must be provided in this positioning operation to tolerate some degree of dimensional or shape change over time that could allow unacceptable contact between the coil or bobbin and a metal pole surface.
A common manufacturing practice has been to manually insert a set of shims of selected thickness between the bobbin and the cylindrical pole piece, leaving them in place until the bonding agent sets. Then, after the shims are removed, a cover member may be adhesively fixed in place over the central region of the diaphragm as a dust seal. Once this is done there is normally no satisfactory way of directly inspecting the finished unit in the factory or later in the field to verify the concentricity or the margin of reserve clearance available: functional testing may reveal instances where there is actual contact but fails to provide any quantitative evaluation of the margin of clearance.
When the bobbin is improperly shaped (out-of-round) or tilted due to improper attachment to the coil assembly, shimming temporarily forces the coil into proper shape and orientation, however when the shims are removed after suspension attachment, the bobbin reverts to its misshape or tilted orientation with the result that the clearance may be reduced to the point of incipient failure even though the actuator may appear to function normally.
There is an unfulfilled need for an automatic method of locating the coil assembly, in preparation for final affixing, without the use of shims so that the coil can rest free in its inherent orientation while it is being optimally centered. This implies the requirement for not only a new method of sensing and optimizing the concentricity during final assembly, but also for a new method of quantitatively evaluating the concentricity that can detect physical anomalies such as out-of-round or tilted coils.